This invention relates to arrays of lenses for focusing thermal infrared radiation, that is, radiation in the wavelength range 8 to 13 microns. More particularly, it relates to an array of lenses for directing and concentrating radiation from a plurality of arcuately displaced directions onto a detector, said lenses being formed as planar radiation concentrators in a sheet of radiation transmissive material. The lenses can be moulded in a thin sheet of plastics material, for example, polyethylene. Although such a material has a relatively poor transmission, absorption loss is low since the sheet is thin. The planar radiation concentrators would most commonly be fresnel lenses. However, the focusing action can be obtained from planar diffracting elements such as zone plates. Such elements can also be moulded in sheet form from a master.
In European patent application 0,197,583 A1 which corresponds to U.S. Pat. No. 4,717,821 issued Jan. 5, 1988, an array of fresnel lenses, moulded in planar form, is described for use in an infrared intruder alarm. The passage of an intruder across any one of the arcuately displaced directions generates a signal in a detector placed to receive the image of the intruder focused by the associated lens. Each direction is defined by the line joining the detector to the pole of the associated lens.
It is desirable in such an intruder alarm that a single detector should cover directions over as wide a range of angles as possible for economy in the number of alarms systems needed to cover any given volume. As the range of angles of the directions becomes larger, the angle between the normal to the sheet surface and the extreme directions becomes larger. Consequently there is an increase in radiation lost by reflection before it can reach the detector, by lens aberrations enlarging the focused image, deflecting radiation away from the detector, and by off-axis lens aperture reduction. Typically, when the angle of incidence of radiation on the sheet is 17 degrees, the lost radiation is half the total. At 33 degrees incidence, the lost radiation is 90 per cent of the total, only 10 percent reaching the detector.
In a typical installation, the alarm is mounted high upon a wall to look out into a volume in directions roughly parallel to the floor. But an intruder may enter the volume by creeping along close to the wall under the alarm. In this case the direction to the intruder will be roughly 70 to 80 degrees to the directions looking out into the protected volume. With a single flat sheet of lenses set at a compromise angle, the radiation loss from the "creep" zone and from distant sources would both be unacceptably large.
However, it is important that the lens array should be a straightforward production proposition in the interest of cheapness. The making of a steel mould to produce the plastic sheet lens array will be greatly simplified and cheapened if the array is composed of a substantially flat sheet.